Add directives for kernel headers and modules to Makefile (follow-up on #159)

[joshbowyer]

Currently there is no simple way to obtain kernel headers or module code for the Luckfox. This is because the Makefiles in this repo dont have any directives to build headers. That is only in the original kernel source, which is not called by the Luckfox Makefiles and uses uncustomized variables.

Ideally, the project maintainers would add a new directive to their Makefile to either include headers, OR (more ideally), link in the bindeb-pkg directive so that .deb installers for the headers and modules can be created, then copied to the Luckfox OS, and installed that way.

I tried figuring out how to do that however I have very little experience with building kernels.

[luckfox-eng33]

Because this follows RK's configuration, we haven't generated kernel headers yet, and may consider adding them later, but they may not be a high priority